Steamflood projects are usually operated at a constant injection rate until the economic limit for steam injection is reached. Subsequently, the injection wells are either converted to hot water injection or are shut-in, and production is continued until project termination.
It is now well recognized that steam overrides in heavy oil reservoirs, especially in thick formations and formations having good vertical communication. This condition results from the fact that vapor phase steam, having a lower specific gravity than oil and water present in the pore spaces of the formation, tends to gravitate toward the upper portion of the formation and to sweep out preferentially this upper portion. Once this has occurred, all the subsequently injected steam tends to follow the same path in the upper portion and to exert little sweeping action on the petroleum-saturated lower portions. This is the condition known as steam override. Furthermore, after steam breakthrough, a significant portion of the injected steam is lost through the production wells, thereby drastically reducing steam utilization. Therefore, regulation of the heat injection rate after steam breakthrough can improve both steam utilization and project economics.
Neuman, in his article "A Gravity Override Model for Steamdrive", J. Pet. Tech. January 1985, pages 163-169, and specifically incorporated herein by reference, first proposed an analytical gravity override model for steamflooding, while also deriving an expression for a steam injection schedule to keep the areal extent of the steam zone constant. Vogel, in his article "Simplified Heat Calculations for Steamfloods", J. Pet. Tech. July 1984, pages 1127-1136, simplified Neuman's model and proposed that the heat injection rate should be sufficient to maintain the rate of vertical steam zone growth and to provide for heat losses. Both the Neuman and Vogel models, however, are essentially heat balance models, thereby limiting their ability to predict oil production rates, and providing no guidelines for an optimum injection schedule.
Additionally, methods to overcome the steam-override condition have been proposed which force steam low into the formation thereby improving vertical conformance. One such method is disclosed in U.S. Pat. No. 4,620,594 to Hall, specifically incorporated herein by reference, which suggests a three dimensional blocking action to obstruct fluid flow within the formation, not merely flow between the formation and the producing well.